The invention relates to a process of preparing a liquid silicon rubber (LSR) composition.
An LSR composition also referred to as a LIM composition, is a multi-component combination of a vinyl-containing polydiorgansiloxane fluid, a hydrogen-containing polydiorganosiloxane fluid, an effective amount of a platinum catalyst and a reinforcing filler, such as a fumed silica and additional additives. Typically, two component mixtures are mixed and charged into a preheated mold where they are rapidly cured to produce a rubber part. A first component mixture includes a vinyl-containing polydiorgansilozane fluid, a silica filler and an effective amount of a platinum catalyst and a second component mixture includes a hydrogen containing polydiorganosiloxane fluid in combination with other ingredients, including a vinyl containing polyorganosiloxane fluid and a silica filler.
Typically, the LSR composition is produced by kneading a polydiorganosiloxane, inorganic filler and additives by means of a kneading machine such as a Banbury mixer, a turbulizer, a change can mixer or a low intensity double arm dough mixer. In this process, polydiorganosiloxane, inorganic filler, treating agents and additives are batch mixed until desired properties are obtained. The batch mixing process can take 12 to 30 hours per batch. After mixing, the LSR composition is stripped of volatiles and cooled. This final step requires additional time to complete preparation of the LSR composition.
A batch process requires long residence times and large amounts of energy. Non-homogeneous shear and extensional stresses across a commercial sized batch can result in non-uniform size distribution of filler that results in variations in properties. Different physical properties may result form batch to batch. Batches processed at different times may be characterized by different physical properties. The batch process is labor, energy and capital intensive and produces materials of only marginal consistency.
There is a need for a continuous process that consistently produces a devolatilized LSR composition from polymer, filler and additives.
The invention provides a continuous process that prepares a devolatilized LSR composition. The process comprises introducing a filler, treating agent and silicone polymer into an extruder having a length to diameter ratio of at least greater than 50. The filler, treating agent and silicone polymer are continuously compounded and devolatilized in the extruder into a devolatilized LSR composition.
In an embodiment, the invention provides a system for preparing a liquid silicone rubber composition. The system comprises a mixer to prepare a densified concentrate of filler and silicone polymer, a long extruder having an L/D ratio of greater than 50, connected to the mixer to receive the densified concentrate from the mixer and to compound and devolatilize the densified concentrate, a treating agent and silicone polymer into a liquid silicone rubber composition containing volatiles and a cooler to receive the liquid silicone rubber composition to cool, homogenize and further devolatilized the composition.